The prior art is represented by DE 10 2004 036 996 B3. According to that, an apparatus for inserting a weft thread into the loom shed is provided on a jet weaving machine, which apparatus consists of a block of main blown nozzles with connected mixing tubes. In each mixing tube there is provided a clamping device by which the weft thread located in the mixing tube is held in a straight oriented manner before its insertion into the loom shed. Each one of these clamping devices comprises an actuator located outside of the mixing tube, which actuator can be pneumatically or piezoelectrically activated and deformed. Thereby, a clamping element connected with the actuator is set into a tilting or pivoting movement. The clamping element engages or reaches into the mixing tube, and with its free end clamps the weft thread against a counter support located on the mixing tube. Preferably the embodiment of the actuator is in the form of an elastomeric bellows, which is directly connected with the clamping element. Through varying pneumatic impingement or activation, the actuator can be activated and deformed, whereby a supply line for a separate pneumatic control fluid is provided.
Besides the differing pressure impingement, also the self-elasticity of the actuator embodied as an elastomeric bellows, an adjusting spring, or a flexible elastic holding element can influence the respective position or adjustment of the clamping element. Finally the clamping element itself can be elastically deformed.
The known clamping devices according to the DE 10 2004 036 996 B3 are already built of module units of respectively two clamping devices; in that regard several module units can be combined together into one block. Nevertheless, relatively little space is available for the individual clamping unit. The holding force of a pneumatically actuated holding device is, however, dependent on its structural size. If the dimensions must be small, the holding force is also limited. The same applies for a hydraulic or piezoelectric activation of the actuators. Additionally, in the case of the pneumatic activation, there is still an interfering influence from the compressibility of the controlling airstreams. That leads to a delayed response behavior of the clamping device. It has been determined, that the clamping element is set into oscillations during the transition from its clamping position into the release position and vice versa, whereby especially fine weft threads are no longer reliably and timely clamped. The delayed response behavior of the clamping devices can already lead to interferences in the weaving process at the rotational speeds of up to 1000 rpm that are typical today in modern weaving machines.
A further pneumatically activated clamping device for holding weft threads on jet weaving machines is known from the JP 2000-119 936 A. There, the clamping element is located similarly like a non-return one-way flap at the outlet of the mixing tube, where it is pivotably secured on a joint located externally on the mixing tube, and is pulled into the outlet opening of the mixing tube by a tension spring that is similarly located externally. In that regard, a weft thread is clamped against a shoulder that forms a part of the outlet opening. The blown air stream of the main blow nozzle that introduces or inserts the weft thread shall open the clamping element against the spring effect and thereby release the clamped weft thread. In this known clamping device, an exact, quickly progressing and exactly controllable transition from the clamping position into the release position cannot be expected, because the control of the clamping element is inseparably associated with the transport of the weft thread.